1. Field of the Invention
This invention relates generally to chemical mechanical planarization apparatuses, and more particularly to methods and apparatuses for improved uniformity in chemical mechanical planarization applications via a membrane based chemical mechanical planarization apparatus.
2. Description of the Related Art
In the fabrication of semiconductor devices, planarization operations, which can include polishing, buffing, and wafer cleaning, are often performed. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. Patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal planarization operations are performed to remove excess metallization. Further applications include planarization of dielectric films deposited prior to the metallization process, such as dielectrics used for shallow trench isolation or for poly-metal insulation. One method for achieving semiconductor wafer planarization is the chemical mechanical planarization (CMP) process.
In general, the CMP process involves holding and rubbing a typically rotating wafer against a moving polishing pad under a controlled pressure and relative speed. CMP systems typically implement orbital, belt, or brush stations in which pads or brushes are used to scrub, buff, and polish one or both sides of a wafer. Slurry is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto a moving preparation surface and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.
FIG. 1A is a diagram showing a conventional table based CMP apparatus 50. The conventional table based CMP apparatus 50 includes a polishing head 52, which holds a wafer 54, and is attached to a translation arm 64. In addition, the table based CMP apparatus 50 includes a polishing pad 56 that is disposed above a polishing table 58, which is often referred to as a polishing platen.
In operation, the polishing head 52 applies downward force to the wafer 54, which contacts the polishing pad 56. Reactive force is provided by the polishing table 58, which resists the downward force applied by the polishing head 52. The polishing pad 56 is used in conjunction with slurry to polish the wafer 54. Typically, the polishing pad 56 comprises foamed polyurethane or a sheet of polyurethane having a grooved surface. The polishing pad 56 is wetted with a polishing slurry having both an abrasive and other polishing chemicals. In addition, the polishing table 58 is rotated about its central axis 60, and the polishing head 52 is rotated about its central axis 62. Further, the polishing head can be translated across the polishing pad 56 surface using the translation arm 64. In addition to the table based CMP apparatus 50 discussed above, linear CMP systems have been conventionally used to perform CMP.
FIG. 1B shows a side view of a conventional linear wafer polishing apparatus 100. The linear wafer polishing apparatus 100 includes a polishing head 108, which secures and holds a wafer 104 in place during processing. A polishing pad 102 forms a continuous loop around rotating drums 112, and generally moves in a direction 106 at a speed of about 400 feet per minute, however this speed may vary depending upon the specific CMP operation. As the polishing pad 102 moves, the polishing head 108 rotates and lowers the wafer 104 onto the top surface of the polishing pad 102.
A platen manifold assembly 110 supports the polishing pad 102 during the polishing process. The platen manifold assembly 110 may utilize any type of bearing such as a fluid bearing or a gas bearing. The platen manifold assembly 110 is supported and held into place by a platen surround plate 116. Gas pressure from a gas source 114 is inputted through the platen manifold assembly 110 via a plurality of independently controlled of output holes that provide upward force on the polishing pad 102 to control the polishing pad profile.
Unfortunately, in each of the above CMP systems non-uniformities in material removal rate and process instability can occur. Generally, uniformity requires all parameters defining the material removal rate to be evenly distributed across the entire contact surface that interfaces with the wafer. In addition, process stability generally requires the properties of the contacting surface to remain essentially constant.
Edge instabilities in CMP are among the most significant performance affecting issues and among the most complicated problems to resolve. FIG. 2 is a diagram showing a wafer pad interface 200, illustrating edge effect non-uniformity factors. As shown in FIG. 2, when the wafer 54 contacts the polishing pad 56 during the CMP process, the flexibility in the polishing pad 56 allows the wafer 54 to form a depression in the polishing pad 56. More particularly, although the polishing pad 56 is a compressible medium, the polishing pad 56 has limited flexibility, which prevents the polishing pad 56 from conforming to the exact shape of the wafer 54, forming transient deformation zones. As a result, edge effects occur at the wafer edge 202 from a non-flat contact field resulting from redistributed contact forces. Hence, large variations in removal rates occur at the wafer edge 202.
Process instability is another problem occurring in prior art CMP systems. Efficient CMP systems allow the use of a given set of consumables for processing of a significant number of wafers, at least several hundred, before the consumables require replacement. Unfortunately, prior art CMP systems inject process instabilities into the CMP process through wear on the polishing surface, as illustrated in FIGS. 3A and 3B.
FIG. 3A is a diagram showing a top view of a prior art table based CMP apparatus 300. As shown in FIG. 3A, when a wafer 54 undergoes planarization using the polishing pad 56, material is eroded from the polishing pad 56 in addition to the material removed from the surface of the wafer 54. Pad material is removed from the polishing pad 56 in a wafer path that contacts the wafer 54 during the CMP process. Moreover, the pad erosion rate is distributed non-evenly, being higher at the central path 304, less near the edge sections 304a of the wafer path, and remaining non-eroded in outer pad regions 302 that are outside the wafer path. As similar behavior occurs on a linear apparatus, as shown in FIG. 3B.
FIG. 3B is a diagram showing a top view of a prior art linear wafer polishing apparatus 350. As shown in FIG. 3B, when a wafer 104 undergoes planarization using the polishing pad 102, material is eroded from the polishing pad 102 in addition to the material removed from the surface of the wafer 104. As above, material is removed from the polishing pad 102 in a wafer path 352 that contacts the wafer 104 during the CMP process. As with table based CMP apparatus 300, the pad erosion rate is distributed non-evenly. The pad erosion rate is higher at the central path 354, less near the edge sections 354a of the wafer path, and remaining non-eroded in outer pad regions 352 that are outside the wafer path.
The variation in the pad material erosion causes the polishing pad to loosen contact with the wafer surface during the CMP process, as shown in FIG. 4. FIG. 4 is a diagram showing a wafer pad interface 400, illustrating pad erosion related instability factors. As mentioned above, material is eroded from the polishing pad in the wafer path and not from outside the wafer path, and more material is eroded from the central path 304 than from the edge sections 304a of the wafer path. As a result, the polishing pad 56 non-evenly loosens its contact with the wafer 54. This loss of contact with the wafer 54 produces not only non-uniformity, but also produces constantly variable non-uniformity depending on the number of wafers processed, which is instability.
Although the air bearing platen approached utilized in a linear wafer polishing apparatus can compensate for the above mentioned non-uniformity and instability in the CMP process, the belt shape of the polishing pad requires complicated manufacturing procedures, which greatly increase the cost. The special manufacturing requirements of the linear wafer polishing belt discourage CMP pad manufactures from actively producing such polishing belts. As a result, linear polishing belts typically have poor quality and reproducibility, which can discourage integrated circuit (IC) manufactures from utilizing the linear wafer polishing systems even though the linear wafer polishing systems may be very well suited for IC fabrication and work well for future technology needs.
In view of the foregoing, there is a need for CMP systems capable of compensating for process instability and non-uniformity. The CMP systems should be capable of compensating for edge effect and other process instability, and should utilize system elements that do not require highly specialized manufacturing equipment.
Broadly speaking, the present invention fills these needs by providing a membrane based CMP apparatus that utilizes a flexible polishing surface and a pad support capable of manipulating the polishing surface shape to compensate for process instability and non-uniformity. In addition, embodiments of the present invention can utilize a conventional pad shape, thus allowing conventional process consumables to be utilized.
In one embodiment, an apparatus for CMP is disclosed. The apparatus includes at least one frame, a polishing membrane attached to the frame, and a pad support disposed below the polishing membrane that is capable of differentially flexing the polishing membrane. The frame can have an essentially round shape, thus allowing a conventional pad shape to be utilized for the polishing membrane. In one aspect, the pad support can provide non-contact reactive force to the polishing pad during a CMP process. For example, the pad support can be an air bearing that provides air pressure to the polishing membrane, or a fluid bearing that provides fluid pressure to the polishing membrane. In a further aspect, the pad support can be in contact with the polishing membrane during CMP. For example, the pad support can include mechanical elements capable of differentially flexing the polishing membrane during a CMP process. Further embodiments of the present invention can include a conditioner element disposed above the polishing membrane, wherein the conditioner element is capable of eroding the polishing membrane. In this embodiment, a conditioner pad support can be included that is disposed below the polishing membrane and the conditioner element.
A method for planarizing a wafer surface during a CMP process is disclosed in a further embodiment of the present invention. A pad support is positioned below the polishing membrane, which is attached to at least one frame. The frame is rotated and downward force is applied to a wafer disposed above the polishing membrane and the pad support. The pad support differentially flexes the polishing membrane while providing reactive force to the polishing pad during a CMP process. As mentioned above, the frame can have an essentially round shape, such as a cylindrical shape. Further, the pad support can be air bearing that provides air pressure to the polishing membrane. As above, the pad support can also include mechanical elements that are in contact with the polishing membrane and capable of differentially flexing the polishing membrane during a CMP process. In addition, the polishing membrane can be eroded during the CMP process using a conditioner element.
A further apparatus for CMP is disclosed in a further embodiment of the present invention. The apparatus includes a cylindrical frame, a polishing membrane attached to an end of the cylindrical frame, and a pad support disposed within the cylindrical frame and below the polishing membrane that is capable of differentially flexing the polishing membrane. As above, the pad support can be air bearing that provides air pressure to the polishing membrane to differentially flex the polishing membrane during a CMP process. In a further aspect, the pad support can be in contact with the polishing membrane, and include mechanical elements that are capable of differentially flexing the polishing membrane during a CMP process. In addition, the apparatus can include a conditioner element disposed above the polishing membrane, and a conditioner pad support disposed below the polishing membrane and the conditioner element, wherein the conditioner element is capable of eroding the polishing membrane.
Advantageously, embodiments of the present invention can compensate for process instability and non-uniformity. Further, the polishing membrane of the embodiments of the present invention can advantageously have a shape and dimensions similar to a conventional table based polishing pad, thus facilitating ease of manufacture using existing or slightly updated manufacturing equipment. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.